In multi-antenna radio transmission technologies based on multiple input multiple output (MIMO), methods for transmitting a directional beam from a base station (BS) in the direction of a mobile station (or user equipment (UE)) are being employed. One example of such methods is adaptive antenna array beam forming (AAA-BF) where a base station estimates the direction of arrival (DOA) of a signal from a mobile station and transmits a directional beam in the estimated direction of arrival.
AAA-BF is described below with reference to FIG. 1. As shown in FIG. 1, a communication system 10 includes a mobile station UE and a base station BS. The base station BS includes four antennas 11A-11D, RF transmission circuits 12A-12D connected to the corresponding antennas 11A-11D, multipliers 13A-13D connected to the corresponding RF transmission circuits 12A-12D, a distributor 14 connected to the multipliers 13A-13D, RF reception circuits 15A-15D connected to the corresponding antennas 11A-11D, multipliers 16A-16D connected to the corresponding RF reception circuits 15A-15D, and a combining unit 17 connected to the multipliers 16A-16D.
In a transmission process at the base station BS, the distributor 14 duplicates a signal and thereby generates four transmission signals, the multipliers 13A-13D multiply the transmission signals by transmitting antenna weights, and the RF transmission circuits 12A-12D perform predetermined processing on the multiplied transmission signals and send the processed signals from the antennas 11A-11D. The antenna weights are obtained based on an estimated direction of arrival of a signal from the mobile station UE. The antenna weights are used to control amplitudes and phases of the transmission signals and make it possible to emit a highly directional beam EW in the direction of the mobile station UE.
In a reception process at the base station BS, signals (or multiple instances of a signal) from the mobile station UE are received at the antennas 11A-11D, the RF reception circuits 15A-15D perform predetermined processing on the received signals, the multipliers 16A-16D multiply the processed signals by receiving antenna weights, and the combining unit 17 combines the multiplied signals to obtain the original signal.
Using a directional beam in a communication system as described above makes it possible to increase the received power at the mobile station and thereby to improve the transmission quality. For details of AAA-BF, see, for example, 3GPP R1-070859, “Adaptive Beamforming in E-UTRA,” NTT DoCoMo, Fujitsu, Mitsubishi Electric, NEC, Sharp; and 3GPP R1-071608, “Beamforming with eight transmit antennas for E-UTRA DL”, Ericsson.